1. Field of the Invention
This invention relates to ablative coatings. More particularly, this invention relates to the use of high vinyl-containing vinylmethylsiloxane polymers as high temperature insulating materials.
2. Description of the Prior Art
A number of rigid, thermally stable materials are suitable for use as ablative insulation. However, corresponding elastomeric materials are not nearly as thermally resistant in the virgin state. The elastomeric materials currently used as ablative insulation are conventional two-part addition-cure RTV silicone rubber materials. They have very low vinyl concentrations, generally less than 0.5 mole percent vinylmethylsiloxane. These materials use a small amount of free vinyl groups prior to cure to form sites for cross-linking during cure. The free vinyl groups are then essentially used up in the cure reaction. Typical conventional elastomers are dimethylsiloxane (DMS) and copolymers such one containing about 30 mole percent methylphenylsiloxane and about 70 mole percent dimethylsiloxane (MPS-DMS). The latter polymer also contains a small amount, about 0.14 mole percent, of vinylmethylsiloxane to facilitate the platinum-catalyzed addition cure reaction to produce the desired gumstock.
The substitution of vinyl groups into a siloxane copolymer is a high vinyl concept which assures a very high concentration of free vinyl groups after cure which are then available for thermally activated cross-linking when exposed to the high thermal flux environment of the operating combustor. For this reason, vinyl substituted polymers are more stable than dimethylsiloxane polymers. The siloxane copolymers exhibit thermal stabilities on the order of 800.degree. F. (427.degree. C.) compared to conventional silicone stabilities of about 450.degree. F. (232.degree. C.). Further, resistance to thermal degradation increases with increasing vinyl content.
An improved lining material with increased erosion resistance, increased resistance to aeroheating loads and increased bonding to propellants is desired. These demands are met by silicone elastomers of high vinyl content which when suitably filled, offer the advantage of forming a nonvolatile char which is itself insulating and helps protect the underlying substrate in very high temperature insulating environments, such as combustion chambers.